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Growing up in the Spy vs. Spy era, and a bit later in the Watergate age, I developed a keen appreciation for clandestine operations, which I assumed at that time were unique to human culture. As it turns out, eavesdropping is practiced by many different species for a variety of reasons. One important example occurs in bird flocks composed of several species of birds. Antshrikes (Thamnomanes ardesiacus) are sentinel species in multi-species flocks because they produce alarm calls when they spot a predacious raptor flying overhead, alerting other nearby birds of the threat. Ari Martinez and his colleagues wondered whether hanging out with antshrikes allowed these other bird species to expand their niches to forage in areas that might otherwise be too dangerous.

An antshrike perched in the Amazonian rainforest. Credit: E. Parra.

This fear-based niche shift hypothesis makes two related predictions. First, in the absence of antshrikes, the remainder of the flock should shift its range to areas with lower predation risk. Second, without antshrikes some birds might leave the flock entirely, because without sentinel services they no longer benefit from hanging with other birds. To test these predictions, Martinez and his colleagues identified eight flocks of 5-8 species (including antshrikes) in a tropical lowland forest in southeastern Peru. They established four removal flocks from which they removed all antshrikes after capturing them in mist nets. They left four control flocks, in which they captured all antshrikes, but then returned them to the flock (to control for the effects of handling).

Research team mist-netting and measuring antshrikes. Credit Micah Reigner

To determine where the flock was spending its time, researchers used a GPS device every 10 minutes to record the center of the flock. They also censused each flock for species composition from dawn to dusk for three days before removal and three days after removal. In control flocks, home range overlapped extensively (average of 69%) when comparing the first (pre-removal) and second (post-removal) three-day period. In removal flocks, there was only 8% overlap in home range, indicating that the remaining flock was shifting its range when antshrikes were gone.

Home ranges of a control flock (top) and a flock which had antshrikes removed (bottom). Red color indicates home range during the three day pre-removal period, while blue color indicates home range during the three day post-removal period. Deeper colors indicate greater occupancy.

But are the remaining species shifting their niches to safer locations when antshrikes are no longer available as sentinels? To answer this question the researchers measured the presence or absence of vegetation cover at different height intervals every 10 minutes at the center of the flock. Comparing the second (post-removal) to the first (pre-removal) period, the removal flocks (those without antshrikes) moved into understory vegetation (0-8 meters high) that was substantially denser than was the vegetation inhabited by the control flocks (those with antshrikes). Presumably, dense understory protects birds without sentinels from being spotted or captured by raptors flying overhead. These dense understory areas are usually associated with less tree cover at higher height intervals (above 16 meters), which allows more sunlight to reach the forest floor, resulting in lush vegetation growth.

Proportion change in vegetation cover occupied by flocks from pre-trial to post-trial period at different height intervals. Positive numbers indicate an increase in vegetation density. Error bars are 95% confidence intervals. Data are based on the behavior of four control and four removal flocks.

Flocking occurrence is the proportion of time individuals of a particular species spend in flocks. The fear-based niche shift hypothesis predicts that flocking occurrence should decrease when sentinel species are removed because the benefits of flocking are reduced for the remaining species. When the researchers compared post-removal to pre-removal time-periods, five species showed strong reductions in flocking occurrence for removal flocks in comparison to control flocks, two were unchanged, and one species showed an increase in flocking occurence.

The authors emphasize that though flocking occurrence decreased for most species, the flocks did remain intact, which indicates that there are probably other benefits from flocking besides the opportunity to eavesdrop. There might be safety in numbers – a decrease in individual mortality as group size increases, or the possibility that the remaining flock members do provide some information about imminent predator attacks.

Martinez and his colleagues conclude that sentinels help other bird species succeed in tropical rainforests, thriving in dangerous habitats where they might otherwise fear to tread. These species may provide important ecosystem services, such as dispersing seeds and eating herbivorous insects that threaten plants that are the foundation of these tropical ecosystems.